Inequality condition for real numbers

CAT 2023 Slot 2 · Quantitative Ability · Medium · Algebra

Any non-zero real numbers x,yx, y such that y≠3y \neq 3 and xy<x+3y−3\frac{x}{y} < \frac{x+3}{y-3}, will satisfy the condition

  1. A.

    if y>10y > 10, then −x>y-x > y

  2. B.

    if x<0x < 0, then −x>y-x > y

  3. C.

    if y<0y < 0, then −x>y-x > y

  4. D.

    xy<yx\frac{x}{y} < \frac{y}{x}

Answer

B

Explanation

We are given: x+3y−3−xy>0\frac{x+3}{y-3} - \frac{x}{y} > 0 y(x+3)−x(y−3)y(y−3)>0\frac{y(x+3) - x(y-3)}{y(y-3)} > 0 xy+3y−xy+3xy(y−3)>0  ⟹  3(x+y)y(y−3)>0\frac{xy + 3y - xy + 3x}{y(y-3)} > 0 \implies \frac{3(x+y)}{y(y-3)} > 0

So x+yy(y−3)>0\frac{x+y}{y(y-3)} > 0.

If x<0x < 0:

  • If y>3y > 3, then y(y−3)>0  ⟹  x+y>0  ⟹  y>−x>0y(y-3) > 0 \implies x + y > 0 \implies y > -x > 0. In this case, −x<y-x < y.
  • If 0<y<30 < y < 3, then y(y−3)<0  ⟹  x+y<0  ⟹  −x>yy(y-3) < 0 \implies x + y < 0 \implies -x > y.
  • If y<0y < 0, then y(y−3)>0  ⟹  x+y>0  ⟹  x>−yy(y-3) > 0 \implies x + y > 0 \implies x > -y. But since x<0x < 0 and y<0y < 0, −y>0-y > 0, so x>−yx > -y is impossible.

Thus, if x<0x < 0, yy must lie in (0,3)(0, 3), which forces −x>y-x > y.

Hence, Option B ("if x<0x < 0, then −x>y-x > y") is correct.

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